Radiofrequency (RF) ablation is a treatment modality that kills unwanted tissue by heat. Starting with cardiac arrhythmia treatment in the 1980s, RF ablation has found clinical application in a number of diseases, and is now the treatment of choice for certain types of cardiac arrhythmia, and certain cancers. During RF ablation, an electrode is inserted into proximity with the target region under medical imaging guidance. Tissue surrounding the electrode in the target region is destroyed by heating via RF electric current.
RF ablation is typically performed at continuous power levels of the order of 20-50 watts, with a contact force of approximately 10 g, and under irrigation. The time of ablation, depending on the size of the lesion to be achieved, is typically approximately 1 minute. In general, higher power levels reduce the time needed for forming a specific lesion. However, in prior art systems large values of continuous power cannot be used because of the danger of steam pops being formed.
US Patent Application 2010/0057072, to Roman et al., whose disclosure is incorporated herein by reference, describes an ablation catheter for performing tissue ablation. The disclosure states that RF energy may be safely delivered potentially at wattages up to 100 W.
U.S. Pat. No. 7,207,989, to Pike Jr. et al., whose disclosure is incorporated herein by reference, describes a method for ablating tissue in or around the heart to create an enhanced lesion. The distal end of a needle electrode is introduced into the tissue. An electrically-conductive fluid is infused through the needle electrode and into the tissue. The tissue is ablated after and/or during introduction of the fluid into the tissue.
US Patent Application 2015/0272655, to Condie et al., whose disclosure is incorporated herein by reference, describes a system for preventing unintended tissue damage from the delivery of unintended bipolar radiofrequency energy. The disclosure states that if 100 watts of RF energy is being delivered but only 10 watts is required to produce a desired electrode temperature, an electrode may be activated for 10% of a given period of time and deactivated for 90% of that duration of time.
U.S. Pat. No. 8,641,705, to Leo et al., whose disclosure is incorporated herein by reference, describes an apparatus for controlling lesion size in catheter-based ablation treatment. The apparatus measures the force exerted by a contact ablation probe on a target tissue and integrates the force over an energization time of the ablation probe. The force-time integral can be calculated and utilized to provide an estimated lesion size (depth, volume and/or area) in real time.
U.S. Pat. No. 8,882,761, to Desai, whose disclosure is incorporated herein by reference, describes a catheter for ablation. The disclosure refers to commonly practiced ablation procedure, and states that in such a procedure 35 to 50 watts of power is delivered at 40 to 50 degree Celsius through a temperature controlled Radiofrequency Generator, and that the saline irrigation fluid rate during the ablation is 30 ml/min.
US Patent Application 2011/0009857, to Subramaniam et al., whose disclosure is incorporated herein by reference, describes an open-irrigated catheter with turbulent flow. Pressurized fluid is delivered from a fluid lumen of a catheter body into an ablation electrode. Fluid flow in the fluid lumen is generally laminar. The generally laminar fluid flow is transformed from the fluid lumen into a turbulent fluid flow within the ablation electrode.
In an article by Topp et al., entitled “Saline-linked surface radiofrequency ablation: Factors affecting steam popping and depth of injury in the pig liver,” Ann. Surg., vol. 239, no. 4, pp. 518-27 (2004), the authors claim to have determined parameters that predict steam popping, and depth of tissue destruction under nonpopping conditions. The article is incorporated herein by reference.
Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that, to the extent that any terms are defined in these incorporated documents in a manner that conflicts with definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.